The present invention relates to a braking device and more specifically to a braking device performing a motion stabilizing control generating a braking force to improve the stability of a vehicle in accordance with vehicle running conditions when a driver is not performing a brake operation.
A conventional example of the above-mentioned braking device is shown in U.S. Pat. No. 5,015,043 issued to Resch (Japanese Examined (KOKOKU) Publication No. 7(1995)-80445).
The conventional braking device has an anti-lock brake system for preventing wheel lock. The anti-lock brake system comprises a fluid pressure control valve and a return pump. The fluid pressure control valve comprises an inlet valve and an outlet valve. The inlet valve and outlet valve control a wheel cylinder pressure in a brake circuit. The brake circuit connects a master cylinder with a wheel cylinder. The return pump returns the brake fluid drained from the outlet valve to an upstream section (which is closer to the master cylinder than the fluid control valve). In this conventional braking device, a pressure pump is arranged for supplying the brake fluid of the master cylinder to the suction side of the return pump. A normally-closed inlet control valve which is opened during the motion stabilizing control is arranged in a suction circuit for connecting the pressure pump with the master cylinder. A normally-closed gate valve which is closed during the motion stabilizing control is arranged between the fluid pressure control valve and the master cylinder. A relief valve is arranged in a bypass circuit arranged in parallel with the gate valve. The relief valve relieves the fluid pressure of the fluid pressure control valve's side into the master cylinder's side by opening the relief bypass circuit when the fluid pressure of the fluid pressure valve's side in the brake circuit is higher than the fluid pressure of the master cylinder's side.
However, the above-mentioned conventional braking device has a following problem, because the relief valve is opened by a differential pressure between the brake fluid pressure of the downstream side(the wheel cylinder's side) of the relief valve of the brake circuit and the brake fluid pressure of the upstream side (the master cylinder's side) of the relief valve.
For example, the motion stabilizing control is a control for controlling drive wheel slip or for improving directional stability of a vehicle by controlling braking forces. The control system restrains drive wheel slippage by generating a braking force on one or more drive wheels, and generates a yaw moment in a direction to stabilize the vehicle motion during cornering by generating a braking force on one or more wheels. In these situations, a driver usually does not apply a brake. Therefore, a brake fluid pressure for this brake control operation is generated in such a manner that an auxiliary supercharging pump supplies the brake fluid from the master cylinder to the return pump, the return pump supplies the brake fluid to the upstream side of the fluid pressure control valve of the brake circuit and the fluid pressure control valve supplies the brake fluid pressure into a wheel cylinder of a desired wheel. In this case, because the inlet control valve is opened and the gate valve is closed, the brake circuit section into which the brake fluid is discharged from the return pump is closed by the fluid pressure control valve, the gate valve and the relief valve. If a pressure difference between the fluid pressure of this circuit section and the fluid pressure of the upstream side of the brake circuit becomes higher than a predetermined pressure, the relief valve is opened to protect the brake circuit by relieving the fluid pressure toward the master cylinder.
However, when the driver applies a brake during the motion stabilizing control, the driver's operation increases the master cylinder pressure and exerts influence on the characteristic of the relief valve responsive to the pressure difference between the upstream and downstream sides. The increase in the master cylinder pressure increases the valve opening pressure (or cracking pressure) at which the relief valve is opened. This increase of the valve opening pressure increases the load on the return pump, increases the required capacity of the motor to meet the increase of the load, increases the required pressure tightness of the control valve and the fluid pressure control valve, and eventually increases the system size and manufacturing cost.